[MEDIUM] add support for configuration keyword registration

[MEDIUM] add support for configuration keyword registration

Any module which needs configuration keywords may now dynamically
register a keyword in a given section, and associate it with a
configuration parsing function using cfg_register_keywords() from
a constructor function. This makes the configuration parser more
modular because it is not required anymore to touch cfg_parse.c.
Example :

static int parse_global_blah(char **args, int section_type, struct proxy *curpx,
                             struct proxy *defpx, char *err, int errlen)
{
	printf("parsing blah in global section\n");
	return 0;
}

static int parse_listen_blah(char **args, int section_type, struct proxy *curpx,
		      struct proxy *defpx, char *err, int errlen)
{
	printf("parsing blah in listen section\n");
	if (*args[1]) {
		snprintf(err, errlen, "missing arg for listen_blah!!!");
		return -1;
	}
	return 0;
}

static struct cfg_kw_list cfg_kws = {{ },{
	{ CFG_GLOBAL, "blah", parse_global_blah },
	{ CFG_LISTEN, "blah", parse_listen_blah },
	{ 0, NULL, NULL },
}};

__attribute__((constructor))
static void __module_init(void)
{
	cfg_register_keywords(&cfg_kws);
}